1. Field of the Invention
This invention relates to a method for compensating modal dispersion in an optical fiber transmission path configured by using multimode optical fibers. The method is directed to reducing modal dispersion of the optical transmission path and obtaining a broad transmission band.
2. Related Art
Multimode optical fibers are widely used in local area networks (LAN) and the like that link computers or related devices in a transmission path, such as factory automation (FA) and office automation (OA) systems. Multimode fibers enable the diameter of the acceptance surface to be increased, and facilitate coupling with light-emitting and light-receiving devices.
One type of optical fiber is a graded index optical fiber (hereinafter referred to as “GI-fiber”) in which the refractive index profiles set such that the refractive index gradually decreases as the radius of the core increases, synchronizing the arrival times of light signal in each mode, has little modal dispersion and obtains a broad band. Since GI fibers obtain a broad band, they are used in optical transmission paths in LAN and the like to increase transmission speed.
One international standard for LANs is Ethernet®. Recently, an Ethernet® with a transmission rate of 10 Gigabits/s (hereinafter referred to as “10 GbE”) has been stipulated, and is expected to be put into practical use. As described in the document below, in order to realize 10 GbE there is a demand to greatly reduce modal dispersion along the transmission path and obtain a broad transmission band. “IEC 60793-2-10 Optical Fibers Part 2–10: Product specifications—Sectional Specification for category A1 multimode fibers”, International Electrotechnical Commission March 2002, pp. 3–30.
Since the transmission band of a GI fiber is determined by its refractive index dispersion, to realize a broad band such as that required by 10 GbE, the refractive index dispersion of the core must be adjusted with extreme precision so that the refractive index has its maximum at the core center and decreases gradually as a function of the radial distance from the center.
Consequently, the range of tolerance for error in the refractive index of GI fiber is extremely narrow, making it difficult to manufacture with good yield and increasing the manufacturing cost. Since the range of tolerance for error in the refractive index is extremely narrow, it is difficult to achieve a stable and constant refractive index dispersion and difficult to manufacture long GI fibers.
Since the range of tolerance for error in the refractive index profile is narrow, it is difficult to form a stable and constant refractive index profile and difficult to achieve the intended broad band. Moreover, since the refractive index profile that obtains the maximum band differs according to the wavelength of the signal light, a broad band cannot be obtained by using a light transmission path that uses conventional GI fibers to propagate signal light in a different waveband to that of the transmission path specification.
As described in the document below, when the refractive index profile of the GI fiber does not satisfy the required profile and a broad band cannot be obtained, a GI fiber having a different refractive index profile from that of the GI fiber (fiber to be compensated) is coupled to the fiber to be compensated as a mode dispersion-compensating fiber to compensate for modal dispersion.
However, this document does not explicitly state specific conditions for compensating modal dispersion, and, for example, proposes no optimum values for the refractive index profile of the mode dispersion-compensating fiber for efficiently compensating modal dispersion of the compensated fiber. W. F. Love, Proceedings of European Conference on Optical Communication '81 (Denmark), Electromagnetics Institute, Technical University of Denmark, 1981, Vol. 4, pp. 4-1 to 4-4.
When using a mode dispersion-compensating fiber to compensate modal dispersion in a GI fiber, Published Japanese Translation No. 2001-52205 of the PCT International Application (WO99/22471) proposes the following method to calculate the length ratio of the compensated fiber and the modal dispersion compensating fiber to be coupled thereto.
The proposed method approximates the refractive index profile of the mode dispersion-compensating fiber and the refractive index profile of the compensated fiber using Equation (1) below, and calculates the refractive index profile exponential parameter α for each.
                              n          ⁡                      (            r            )                          =                  {                                                                                                                n                      1                                        ⁡                                          [                                              1                        -                                                  2                          ⁢                          Δ                          ⁢                                                                                                          ⁢                                                                                    (                                                              r                                /                                a                                                            )                                                        α                                                                                              ]                                                                            1                    /                    2                                                                                                (                                      0                    ≦                    r                    ≦                    a                                    )                                                                                                                                                n                      1                                        ⁡                                          [                                              1                        -                                                  2                          ⁢                                                                                                          ⁢                          Δ                                                                    ]                                                                            1                    /                    2                                                                                                (                                      a                    <                    r                                    )                                                                                        (        1        )            wherein n(r) is the refractive index at a distance r from the core center of the optical fiber, n1 is a refractive index at the core center, Δ is a relative refractive index difference of the core center with respect to a cladding, a is a core radius, and α is a refractive index profile exponential parameter.
Using respective refractive index profile exponential parameters α, the length of the mode dispersion-compensating fiber is determined so that the length ratio of the mode dispersion-compensating fiber and the compensated fiber satisfies Equation (2) below.
                                          L            comp                                L            target                          =                                            α              target                        -                          α              opt                                                          α              opt                        -                          α              comp                                                          (        2        )            where Lcomp is a length of the mode dispersion-compensating fiber, Ltarget is a length of the compensated fiber, αcomp is a refractive index profile exponential parameter of the mode dispersion-compensating fiber, αtarget is a refractive index profile exponential parameter of the compensated fiber, and αopt is the refractive index profile exponential parameter in a refractive index profile that obtains the maximum band at a predetermined wavelength.
The conventional method for calculating the length of mode dispersion-compensating fiber uses empirical rules and produces many errors. Additional errors arise from the need to approximate refractive index profiles of the mode dispersion-compensating fiber and the compensated fiber with the above Equation (1). Consequently, modal dispersion cannot be sufficiently compensated and it is difficult to achieve a broad band appropriate for high-speed transmission.
Another method proposes calculating optimum values for the length ratio Lcomp/Ltarget of the mode dispersion-compensating fiber and the compensated fiber by calculating a group velocity of the signal light propagating through the mode dispersion-compensating fiber and the compensated fiber using electromagnetic field analysis, and then making a simulation based on the calculated values.
However, this method requires complex computation, and a program for computation is not easy to develop. Since a long time is needed to compute the simulation, this method cannot be used in sites where an optical transmission path is laid.